Petroleum refining



Dec- 15, 1959 R. J. HENGsTEBEcK 2,917,450

PETROLEUM REFINING Filed Nov. 29. 1957 INVNTOR (ngsfebeck ,m

ATTORNEY Robert J. Hel"y PETROLEUM REFINING Robert J. Hengsteheck,Valparaiso, Ind., assignor to ntndard Oil Company, Chicago, Ill., acorporation of lana Application November 29, 1957, Serial No. 699,781

Claims. (Cl. 208-66) This invention relates to a process for conveninggas oil to naphtha of high octane, and it particularly concerns a seriesof integrated processing steps to achieve this purpose.

The widespread use of naphtha hydroforming using platinum-type catalystshas provided petroleum reners withy substantial amounts of hydrogen.These amounts of hydrogen are generally insulicient to satisfy thereliners need for hydrogen in more recently proposed processes for thehydrogen treating of various refinery oil streams.

An object of the present invention is to provide a refining process forconverting gas oil to high octane naphtha in substantial yields and atthe same time producing hydrogen for use within the process or for usewithin other processes. Another object is to provide an integratedsequence of processing steps by which gas oil is converted to naphtha ofhigher than ordinary octane number. A further object is to provide anintegrated process for converting gas oil to high octane naphtha whichminimizes the conversion of oil to coke and gas and thereby obtains highyields of naphtha per barrel of gas oil charge. Other objects andadvantages of the present invention will be apparent from the detaileddescription thereof.

In accordance with the present invention a gas oil is dehydrogenated inthe presence of a hydrogenation-dehydrogenation catalyst at temperaturesin the range of 750 to 950 F. and in the presence of recycled hydrogen.Substantial amounts of hydrogen are produced. The gas oil is renderedmore unsaturated and minor amounts of high octane naphtha are formed.This unsaturated oil, preferably after the removal of naphtha, is thensubjected to mild catalytic cracking. The conditions employed in thecracking step are such that no more than about 30 to 40% of thedehydrogenated gas oil charge is converted into naphtha and otherproducts which boil below 400 F. A catalytic cycle oil is recovered fromthe products of the mild cracking step and is subjected to solventextraction with a solvent which preferentially extracts aromatichydrocarbons therefrom. Thereby, an aromatics-rich hydrocarbon extractoil is separated from an aromatics-lean hydrocarbon raliinate oil. Theraflinate oil is subjected to severe catalytic cracking so that at least60% of the charged oil is conveited into naphtha and other hydrocarbonswhich boil below 400 F. The aromatics-rich extract oil is subjected todestructive hydrogenation by contacting it With a catalyst havinghydrogenation and cracking properties in the presence of 2000 to 10,000s.c.f. of hydrogen/barrel of extract oil While employing a temperatureof 800 to l000 F. and a pressure of 1000 to 5000 p.s.i.g. Hydrogenproduced in the dehydrogenation step is employed as a source of hydrogenin the destructive hydrogenation step. Naphtha of very high octanenumber is produced together with some incompletely converted butpartially hydrogenated oil boiling above 400 F. This partiallyhydrogenated oil may be recycled to the destructive hydrogenation stepand/or it may be passed as part of the charge oil to the solventextraction Step. The catalytic cycle oil produced in the severecatalytic cracking step may be recycled thereto or preferably it ispassed as part of the charge oil to the solvent extraction step.

At least a major portion of the gas oil initially charged to the processshould be virgin gas oil which preferably has an end boiling point of550 to 650 F. The higher boiling portion of the gas oil may be chargeddirectly to the severe catalytic cracking Step. When the initial gas oilcharge to the dehydrogenation step has a high end boiling point of 800F. or higher or an ASTM 50% point of 600 F. or higher, then thepartially hydrogenated oil boiling'above 400 F. which is produced in thedestructive hydrogenation step may desirably be passed as charge oil tothe severe catalytic cracking step.

In the dehydrogenation step, substantial amounts of hydrogen areproduced which are necessary in the later destructive hydrogenationstep. By restricting the charge gas oil to the dehydrogenation step to avirgin gas oil hav ing an end boiling point of about 550 to 650 F., cokeformation upon the dehydrogenation catalyst and deactivation of thecatalyst is reduced. By carrying out the mild catalytic cracking stepunder conditions such that no more than 30 to 40% conversion isobtained, coking upon the cracking catalyst and deactivation thereof areminimized. Because of the previous dehydrogenation step, the naphthaproduced from mild catalytic cracking is of higher than ordinary octanenumber. This is partly due to its increased olefinic content and themore aromatic nature of the naphtha. Solvent extraction of the cycle oilfrom the mild catalytic cracking step segregates the cycle oil intofractions which are optimum for the subsequent processing steps. Therainate oil is catalytically cracked under severe conditions withoutsubstantial coke formation upon the catalyst, since substantial amountsof aromatics have been removed during the extraction step. Thedestructive hydrogenation step produces very high octane gasoline on theorder of .F-l or higher from the aromatics-rich extract phase, whereasthe octane number would be substantially lower if extraction had notbeen carried out. By recycling the catalytic cycle oil from the severecatalytic cracking step as charge to the solvent extraction step, and byrecycling as charge to the extraction step the partially hydrogenatedoil boiling above 400 F. which iS obtained from the products of thedestructive hydrogenation step, these recycle streams are againsegregated into optimum streams for catalytic cracking and destructivehydrogenation. When the initial charge to the dehydrogenation step ofthe process is a high boiling gas oil, e.g. having an ASTM 50% point of600 F. or higher, the partially hydrogenated oil boiling above 400 F.which is recovered from the destructive hydrogenation step may be passedto the severe catalytic cracking step because higher octane gasoline,e.g. 95 F-l would be obtained therefrom than would be obtained bydestructive hydrogenation thereof. Because it is partially hydrogenatedit is improved as a charge to catalytic cracking. Also, if it weredestmctively hydrogenated then the conversion would be much less thanthe conversion of a lower boiling fraction and the octane numberproduced from this high boiling fraction would only be about 88-90 F-l.It is is apparent that many benefits are provided by integration of theparticular sequence of refining steps.

The gure shows in diagrammatic form an embodiment of the presentinvention whereby gas oil is converted to high octane naphtha fractionsin high yield. Numerous pumps, heaters, coolers, and other incidentalfeatures which are apparent to those skilled in the art have beenomitted from this figure to `are dehydrogenated in part.

v kieselguhr, fullers earth or the like. cobalt oxide-molybdenumoxide-alumina catalyst contain- 3 obtain improved clarity of thefundamental features of the invention.

In this embodiment a crude oil is charged from source l11 by way ofline-12 into fractionation system represented herein by vessel 13.Naphthaand lower boiling hydrocarbons are removed This virgin oil isfractionated.

from the system by way of line 14 and passed to refining means not shownherein. Residuum is removed by way of line 16. The gas oil isfractionated at a cut point of about 600 F. The higher boiling portionis removed from fractionating means 13 by way of line 17 and isprocessed by means which will be discussed later. The lower boilingfraction of the gas oil such as boils substantially within the range of400 or 450 F. to about 600 or 650 F. is removed from fractionating means13 and passed by way of line 18 together with recycled hydrogen throughfurnace 19 wherein the mixture is heated to the temperatures employed indehydrogenation.

. of groups 6a and/or 8 of the periodic table carried upon a supportsuch as alumina, bauxite, or clays such as For example, a

ing about 3% cobalt oxide and 9% molybdenum oxide may suitably be used.'Ihe dehydrogenation reaction is carried out in the presence ofhydrogen, usually in the amount of from `1000 to 4000 s.c.f./barrel ofoil charged in order to suppress the formation of coke upon thecatalyst. Conditions of temperature and pressure which are conducive todehydrogenation are employed. Such temperatures may be from 750 to 950F. and pressures of from 200 to 800 p.s.i.g. Space velocities of fromabout Y 0.2 to volumes of oil/hour/volume of catalyst may be employed.

In the embodiment described herein a series of three dehydrogenationreactors designated as reactors 22, 23,

and` 24 are employed. The particular arrangement enables the maintenanceof a more nearly constant temperature within each dehydrogenationreactor.

Because the reaction is endothermic, the arrangement shown in thisembodiment prevents Wide variation in reaction temperatures Within thereactors and enables operation at more nearly the optimum temperature.The heated light gas -oil and hydrogen are passed by way of line 21 fromfurnace 19 into dehydrogenation reactor 22. Additional hydrogen may berecycled from the hydrogen separator and passed into line 27. Separatemeans for preheating hydrogen to the reactor temperatures may beemployed in the process if desired. The reaction products from reactor22 are removed therefrom and passed by way of line 27 through furnace 28where the mixture is heated. The heated mixture of partiallydehydrogenated oil and hydrogen is removed from furnace 28 by way ofline 29. The mixture is passed by way of line 31 into dehydrogenationreactor 23. The total reaction products from reactor 23 are withdrawn byway of line 33. Recycled hydrogen may be introduced into line 33. Themixture passes by way of line 33 into furnace 34 wherein it is heated tothe reaction temperature. The heated mixture is removed from the furnaceand passed by way of line 36 into dehydrogenation reactor 24. The totalreaction products are removed from reactor 24 and passed by way of line37 into gas separator 38. A hydrogen stream is stream is recycled by wayof line 41 and valved lines 42 and 43 to the various dehydrogenationreactors.

a' -i iatevzftzso-l A liquid stream is removed from the bottom of gasseparator 38 and passed by way of line 44 into fractionator 46. A smallquantity of a naphtha fraction is recovered therefrom and passed by wayof line 47 to further retining or to blending as shown herein. Lightergases are removed from fractionator 46 by way of line 48. Thedehydrogenated gas oil boiling range material, i.e. the oil boilingabove 400 F. is removed as a bottom stream from fractionator 46 andpassed by way of line 49 through the heating tubes of furnace 51. Thedehydrogenated gas oil, from which a substantial portion of the sulfurand nitrogen compounds have been removed by virtue of thedehydrogenation step, is heated in furnace 51 to the temperaturesemployed in the mild catalytic cracking step. The heated oil is passedfrom the furnace by Way of line 52 into reactor 53 wherein the mildcatalytic cracking step is carried out.

The dehydrogenated gas oil is catalytically cracked -under conditions toachieve a conversion of from 30 to 40% thereof into lower boilinghydrocarbon products. These cracking conditions may comprise atemperature of from 850 to 1050 F., a pressure of 5 to 50 p.s.i.g., acatalyst to oil ratio in the range of about 2:1 to 20:1 on a Weightbasis, and a Weight space velocity in the range of about 1.0 to 30pounds of oil/hour/pound of catalyst. The particular space velocity,temperature, etc. may vary somewhat depending upon the activity of thecracking catalyst used. Nevertheless, the conversion should be limitedto 30 to 40%, for at higher levels of conversion excessive formation ofcarbonaceous deposits on the cracking catalyst will occur. Either alluidized, moving, or fixed bed catalytic cracking reactor may be used.Siliceous cracking catalysts such as natural clay, activated naturalclay, synthetic catalysts such as silicaalumina, silica-magnesia,silica-alumina-zirconia, etc. may be employed. In the embodiment shownherein the dchydrogenated light gas oil is catalytically cracked using asilica-alumina catalyst, a temperature of about 950 F., a pressure ofabout 25 p.s.i.g., a catalyst to oil weight ratio of about 5-l0, and aspace velocity of about 10 pounds of oil/hour/pound of catalyst in thelluidized reactor. A conversion of about 35% of the gas oil charge intolower boiling products, principally high octane oletinic naphtha, isobtained.

The total reaction products are recovered from the mild catalyticcracking step and passed by way of line 54 into gas separator 55. Lightgases are removed from the system by way of line 56. The liquid ispassed from the separator by way of line 57 into fractionator 58. Thehigh octane olenic naphtha is separated and passed by Way of line 59 toline 47 wherein it is blended with the naphtha from the dehydrogenationstep. The catalytic cycle oil, i.e. oil which boils above 400 F. andwhich is recovered from the products of the mild catalytic crackingstep, is removed from the bottom of fractionator 58 and passed by way ofline 61 as charge oil to the solvent extraction step.

, vessel 62 employing liquid solvents which are selective l removedtherefrom by way of line 39. A portion of this This catalytic cycle oilhas a rather high aromatics content, e.g. 50 to 60% because of theearlier dehydrogenation step. This high aromatics content makes it anundesirable catalytic cracking charge stock since it would causedeposition of large amounts of coke upon the cracking catalyst and leadto rapid deactivation of the catalyst. The catalytic cycle oil from themild catalytic cracking step, together with catalytic cycle oil from thesevere catalytic cracking step and together with partially hydrogenatedoil boiling above 400 F. from the destructive hydrogenation products, ispassed by way of line '61 into the bottom of solvent extraction vessel62. A typical solvent extraction process is carried out in `are liquidSO?, phenol, cresol, chlorex, furfural, etc.

They may be .used inamounts of from 25 to 200 volume percent based uponoil in a process which employs one or many extraction stages. In theembodiment shown herein liquid SO2 is employed as the selective solventat an extraction temperature of about l to 25 C., eg. about 15 C. andunder suiicient pressure to maintain the SO2 in the liquid phase. Theextraction is carried out in three stages, employing about 50 volumepercent of liquid SO2 based upon oil in each stage. ln the schematicdiagram shown in the iigure liquid SO2 :from source 63 is passed by wayof line 6d into the top of extraction vessel 62. The descending streamof liquid SO2 passes downwardly through the ascending stream of lighteroil and extracts aromatic hydrocarbons, as well as any residual amountsof sulfur compounds, from the catalytic cycle oil.

The raffinate phase which consists primarily of oil which is now lean inaromatics, together with some occluded SO2, is removed from the top ofextraction vessel 62 and passed by way of line 6o into iiash drum 67.SO2 is vented from the oil in flash drum o7 and the SO2 is passed by wayof line 63 into line ed for return to the extraction step. Thehydrocarbon rafnate oil is removed from the bottom of flash drum 57 byway of line 69, freed of residual SO2 by equipment not shown herein, andpassed as the charge stock to the severe catalytic cracking step. Anextract phase consisting of liquid SO2 containing dissolved oil which isenriched in aromatic hydrocarbons, is removed from the bottom ofextraction vessel 62 and passed by way of line 71 into ilash drum 72.SO2, which is iiashed from the extract phase in flash drum 72, is takenoverhead and passed by way of line 73 into line 63 by which it isrecycled to extraction vessel 62. The aromatics-rich hydrocarbon extractphase, which now has an aromatics content of about 80% more or less, isremoved from the bottom of flash drum 72, freed ot residual SO2 byequipment not shown herein, and passed to the destructive hydrogenationstep.

In the destructive hydrogenation step the aromaticsrich hydrocarbonextract is converted to naphtha having an F-l octane number of 95 to100. Because substantially all of the sulfur compounds have been removedduring the dehydrogenation step long catalyst life and unusually highoctane naphtha are obtained during destructive hydrogenation. Thecatalyst employed in destructive hydrogenation is a dual-functioningcatalyst which combines hydrogenation properties and cracking properties so as to cause hydrogenation of the extract oil and thereaftercracking of the oil. The hydrogenation cornponents of such a catalystmay be the oxides and/or sulfides of the metals of group 6 and/ or 8 ofthe periodic table (or the metals themselves). These are supported on acarrier having cracking properties such as natural and activated clays,synthetic catalytic cracking catalysts such as silica-alumina,silica-magnesia, silica-alumina* zirconia, or cracking bases such as HFpromoted alumina. The catalyst may contain between l to 10%, preferably`about or thereabouts by weicht, of the hydrogenation component'supported in extended form upon the cracking component. The catalystmay be prepared by any of the conventional techniques such as byimpregnation of the support with an aqueous solution of thehydrogenation component, by precipitation of the hydrogenation componentupon the cracking support, or by coprecipitation of the hydrogenationcomponent with the cracking component. For example a silica-aluminacracking catalyst containing between 5 and 20% alumina with theremainder being silica, may be impregnated with a solution of ammoniummolybdate, the impreg- -nated catalyst dried and then calcined toconvert the ammonium molybdate to molybdenum oxide; thereby producing alcatalyst containing about 5% MoOS. Other catalysts such as nickel on4silica-alumina, iron on ..-ilica-a1umina,platinum on silica-alumina,rplatinum onv tluorided alumina, cobalt molybdate on fluorided alumina,molybdenum oxide on fluorided terrana earth,.and similardual-functioning catalysts may be employed. This dual-functioningcatalyst converts the polycyclic aromatics in the extract oil to naphthaby hydrogenating `one ring of the polycyclic and thereafter by reason ofthe cracking component of the catalyst this hydrogenated ring is crackedwhereupon the naphtha boiling range monocyclic aromatic is produced. Thedestructive hydrogenaton in the presence of the dual-functioningcatalyst is carried out at a temperature between about 800 to about l000and at a pressure of about 1000 to 5000 p.s.i.g. while employinghydrogen in the amount of about 2000 to 10,000 scf/barrel of charge oil.A space velocity of from l to 20 volumes of oil/hour/volume of catalystmay be used. Conversions to lower boiling products on the order of orhigher are obtained, most of it being naphtha having an octane of to 100F--l or higher.

The extract oil is removed from flash drum 72 by way of line 74. Aportion is passed by way of line 76 through the tubes of furnace '77wherein the oil is heated to the temperature needed for the destructivehydrogenation reaction. The heated oil is then passed by way of line 7Sinto destructive hydrogenation reactor 79. The major portion of thehydrogen stream separated from the products of the dehydrogenation stepis diverted from line 39 and passed by way of line 81 through the tubesof furnace 82 wherein it is heated and then passed by way of line 83into line 78 for introduction with the charge oil to the destructivehydrogenation step. A portion of the extract oil is diverted from line74, and thus bypasses furnace '77, and is passed by way of line 84through a manifolding system and into destructive hydrogenation reactor79. This cool oil stream which is introduced at different heights withinthe reactor assists in minimizing the temperature increase caused by theexothermic hydrogenation reaction. In this embodiment the destructivehydrogenation is carried out at a temperature of about 950 F., apressure of about 3000 p.s.i.g., a hydrogen recycle rateiof about 5000s.c.f./barrel of charge oil, a space velocity of about 5 volumes ofoil/hour/volume of catalyst, while employing a molybdena onsilica-alumina containing about 5% M003 and about 20% A1203. The largequantity of hydrogen generated during the dehydrogenation step provideshydrogen for the 2000 to 3000 s.c.f. of hydrogen consumed per barrel ofoil charged to the destructive hydrogenation step.

The products from the destructive hydrogenation step are removed fromreactor 79 and passed by way of line 86 into gas separation means 87. Ahydrogen gas stream is taken overhead and passed by way of line 88 .intoline S1 by which it is recycled to destructive hydrogenation reactor 79.A liquid bottom stream is removed from separator 87 and passed by way ofline 89 into fractionator 91. The high octane naphtha is removedoverhead and is passed by way of line 92 into line 47 wherein it isblended with the other high octane naphtha. components. A bottom streamis removed from fractionator 91 by way of line 93. This stream consistsof an oil which has been partially hydrogenated during the destructivehydrogenation step, and this oil boils at a temperature above 400 F. Itmay be recycled in whole or in part by way of valved line 94 todestructive hydrogenation reactor 79. It may be fractionated and thatportion thereof which boils above 600 F. may be passed by way of valvedline 96 as a portion of the charge to the severe catalytic cracking stepto be discussed later. This latter operation may suitably be carried outbecause the partially hydrogenated nature of the oil has reduced itscoking tendency upon catalytic cracking and because it would produce ahigher octane naphtha-during catalytic cracking than it would duringdestructive hydrogenation. This latter mode of operation mayalso .beemployed when the gas oil charged to the dehydro- `a very substantial ormajor portion of oil boiling above 600 F. and this total oil in line 93may suitably be passed to the severe catalytic cracking step. In theembodiment discussed herein, it is preferred to pass the partiallyhydrogenated oil boiling above 400 F. by way of line 93 through valvedline 97 into line 61 by which it is recycled to the extraction vessel62. In this manner the aromatic components are eventually returned tothe destructive hydrogenation step and the paralnic components pass outwith the railinate oil and are passed to the severe catalytic crackingstep. This technique enables maximum yields of highest octane naphthafrom the stream contained in line 93.

The raiinate oil which is removed from flash drum 67 by way of line 69is passed by way of line 9S into the furnace tubes of furnace 99. Theheavy gas oil, which boils at a temperature above 600 F., which has beensegregated from the light virgin gas oil in fractionator 13 is passed byway of line 17 into line 98. The mixture of heavy virgin gas oil andraffinate oil which has been depleted in aromatics is heated in furnace99 to the temperatures necessary for subsequent catalytic cracking.Heated oil is removed from the furnace and passed by way of line 101 tosevere catalytic cracking reactor 102'. The oil is cracked under aseverity to provide a conversion of at least 60% thereof intohydrocarbons boiling below 400 F. The conditions under which this iscarried out comprise a temperature of 850 to 1050 F., a pressure of 5 to50 p.s.i,g., a catalyst to oil ratio in the range of about 2:1 to 20:1on a weight basis, and a weight space velocity between about 0.1

and 5 pounds of oil/hour/pound of catalyst. Siliceous cracking catalystssuch as silica-alumina and the other types previously described inconnection with the mild catalytic cracking step are used. While theprecise conditions employed in the severe catalytic cracking step willvary to some extent depending upon the activity of the catalyst,particular conditions which may be used to achieve the conversion of atleast 60% of the.

oil to lower boiling products are easily determined by those skilled inthe art. In the embodiment described herein, the oil is catalyticallycracked to achieve the minimum 60% conversion by employing asilica-alumina catalyst, a temperature of about 950 F., a pressure ofabout 25 p.s.i.g., a catalyst to oil weight ratio of about 10, and aspace velocity of about 0.3 to 0.7 pound of oil/hour/pound of catalyst.Any of the various types of commercial catalytic cracking processes suchas the uidized bed technique employed in this embodiment, moving bed, orfixed bed, etc. may be used.

The hydrocarbon products from the catalytic cracking step are moved fromcatalytic cracking reactor by way of line 103 and passed to gasseparator 104. Light gases are taken overhead and removed from theprocess by way of line 106. The liquid is passed from the gase separatorby way of line 107 into fractionating column 108. High octane naphtha isremoved overhead from the fractionator and passed by way of line 109into line 47 wherein it is blended with the other high octane naphthafractions and the desired amount of light hydrocarbons to form highoctane gasoline. A cycle oil stream is removed from the bottom offractionator 108 by way of line 111. If desired this stream may berecycled by way of valved line 112. into line 17 and thereafter returnedto the catalytic cracking step. In the embodiment shown herein itis'preferred to pass a portion or all of this cycle stock from severecatalytic cracking by way of valved line 113 into line 61 by which it isintroduced as a part of the charge oil to the solvent extraction step.This enables the segregation of the aromatic components from the paraniccom'- ponents of the cycle oil, the aromatic components being passedsubsequently to destructive hydrogenation and the paraffnic componentsbeing returned to the severe catalytic cracking step.

-It is apparent from the foregoing description that the presentinvention provides an integrated system for producing maximum yields ofhigh octane naphtha with minimum coke and gas formation by process whichis self-sulcient with respect to hydrogen requirements.

Thus having described the invention what is claimed is:

1. A process for the conversion of gas oil to naphtha which comprisesdehydrogenating a gas oil in the presence of hydrogen and ahydrogenation-dehydrogenation catalyst at a temperature between about750 and 950 F. and a pressure in the range of about 200 to 800 p.s.i.g.and thereby producing hydrogen and a partially dehydrogenated oil,separating the products from the dehydrogenation step into a hydrogenstream, a naphtha stream, and a dehydrogenated gas oil stream,separating the hydrogen stream into rst and second portions, recyclingthe rst portion of the hydrogen stream to the dehydrogenation step,subjecting said dehydrogenated gas oil stream to mild catalytic crackingunder conditions such that no more than 40% of the dehydrogenated gasoil stream is converted into hydrocarbons boiling below'400 F.,recovering a catalytic cycle oil from the products of the mild catalyticcracking step and solvent extracting said cycle oil to separate anaromatics-rich hydrocarbon extract oil from an aromatics-leanhydrocarbon ratlnate oil, subjecting said rainate oil to severecatalytic cracking under conditions to convert at least 60% of saidraffinate oil into hydrocarbons boiling below 400 F., and subjecting thearomatics-,rich extract oil to destructive hydrogenation by contactingit together with the second portion of the hydrogen stream producedduring the dehydrogenation step with a catalyst having hydrogenation andcracking properties in the presence of 2000 to 10,000 s.c.f. ofhydrogen/barrel of extract oil at a temperature of about 800 to 1000 F.and a pressure between about 1000 and 5000 p.s.i.g. and therebyproducing high octane naphtha and a partially hydrogenated oil boilingabove 400 F.

2. The process of claim 1 wherein the initial gas oil to be converted isfractionated at a cut point no higher than about 600 F. to produce alower boiling gas oil and a higher boiling gas oil, and wherein thelower boiling gas oil is charged to said dehydrogenation step and thehigher boiling gas oil is chargedto the severe catalytic cracking step.

3. The process of claim 1 wherein the products from said severecatalytic cracking step are fractionated to separate a naphtha fractionfrom a second catalytic cycle oil, and said second catalytic cycle oiland said partially hydrogenated oil boiling above 400 F. which isproduced during the destructive hydrogenation step are passed as part ofthe charge oil to the solvent extraction step.

4. The process of claim 1 wherein the gas oil charged to thedehydrogenation step has an ASTM 50% point of at least 600 F. andwherein the partially hydrogenated oil boiling above 400 F. which isproduced during the destructive hydrogenation step is passed to thesevere catalytic cracking step.

5. A process for the conversion of gas oil to naphtha which comprisesfractionating a virgin gas oil at a cut point no higher than about 600F. and thereby producing a lower boiling gas oil fraction and a higherboiling gas oil fraction, contacting said lower boiling gas oil fractionin the presence of hydrogen and a hydrogenation-dehydrogenation catalystat a temperature between about 750 and 950 F. and a pressure in therange of about 200 to 800 p.s.i.g. and thereby producing hydrogen and apartially dehydrogenated oil, separating the products from thedehydrogenation step into a hydrogen stream, a naphtha stream, and adehydrogenated gas oil stream, separating the hydrogen stream into[first and second portions, recycling the iirst portion of the hydrogenstream to the dehydrogenation step, subjecting said dehydrogenated gasoil stream to mild catalytic cracking under conditions such that no morethan 40% of the dehydrogenated gas oil stream is converted intohydrocarbons boiling below 400 F., recovering a rst catalytic cycle oilfrom the products of the mild catalytic cracking step and solventextracting said cycle oil to separate an aroniatics-rich hydrocarbonextract oil from an aromatics-lean hydrocarbon rainate oil, subjectingsaid ranate oil and said higher boiling virgin gas oil fraction tosevere catalytic cracking under conditions to convert at least 60% ofthe oil into hydrocarbons boiling below 400 rF., fractionating theproducts from said severe catalytic cracking step to separate a naphthafraction from a second catalytic cycle oil, passing said secondcatalytic cycle oil as charge oil to the solvent extraction step,subjecting the aromatics-rich extract oil to destructive hydrogenationby contacting it together with the second portion of the hydrogen streamproduced during the dehydrogenation step with a catalyst havinghydrogenation and cracking properties in the presence of 2000 to 10,000s.c.f. of hydrogen/ barrel of extract oil at a temperature of about 800to 1000 F. and a pressure between about 1000 and 5000 p.s.i.g. andthereby producing high octane naphtha and a partially hydrogenated oilboiling above 400 F., and passing said partially hydrogenated oilboiling above 400 F. as ycharge oil tothe solvent extraction step.

References Cited in the le of this patent UNITED STATES PATENTS2,242,504 Benedict et al May 20, 1941 2,249,584 Thomas July 15, 19412,748,055 Payne May 29, 1956

1. A PROCESS FOR THE CONVERSION OF GAS OIL TO NAPHTHA WHICH COMPRISESDEHYDROGENATING A GAS OIL IN THE PRESENCE OF HYDROGEN AND AHYDROGENATION-DEHYDROGENATION CATALYST AT A TEMPERATURE BETWEEN ABOUT750* AND 950*F. AND A PRESSURE IN THE RANGE OF ABOUT 200 TO 800 P.S.I.G.AND THEREBY PRODUCING HYDROGEN AND A PARTIALLY DEHYDROGENATED OIL,SEPARATING THE PRODUCTS FROM THE DEHYDROGENATION STEP INTO A HYDROGENSTREAM, A NAPHTHA STREAM, AND A DEHYDROGENATED GAS OIL STREAM SEPARTINGTHE HYDROGEN STREAM INTO FIRST AND SECOND PORTIONS, RECYCLING THE FIRSTPORTION OF THE HYDROGEN STREAM TO THE DEHYDROGENATION STEP, SUBJECTINGSAID DEHYDROGENATED GAS OIL STREAM TO MILD CATALYTIC CRACKING UNDERCONDITIONS SUCH THAT NO MORE THAN 40% OF THE DEHYDROGENATED GAS OILSTREAM IS CONVERTED INTO HYDROCARBON BOILING BELOW 400* F., RECOVERING ACATALYTIC CYCLE OIL FROM THE PRODUCTS OF THE MILD CATALYTIC CRACKINGSTEP AND SOLVENT EXTRACTING SAID CYCLE OIL TO SEPARATE AN AROMATICS-RICHHYDROCARBON EXTRACT OIL FROM AN AROMATICS-LEAN HYDROCARBON RAFFINATEOIL, SUBJECTING SAID RAFFINATE OIL TO SEVERE CATALYTIC CRACKING UNDERCONDITIONS TO CONVERT AT LEAST 60% OF SAID RAFFINATE OIL INTOHYDROCARBON BOILING BELOW 400*F.,